The present invention pertains to an electrical supply system for an internal combustion engine having a flywheel-mounted permanent magnet alternator supplying a voltage regulator utilizing silicon controlled rectifiers (SCRs) and diodes connected as an SCR/diode bridge rectifier for controlling the current supplied to the DC portion of the electrical system. More particularly, the invention relates to a protective circuit means for assuring that the protective crowbar SCR device, utilized to protect the DC portion of the electrical system from excessive regulator output voltage, is allowed to cool off and adequately recover after it has been triggered by an overvoltage surge to shunt the associated alternator output current to ground.
The use of an SCR/diode bridge rectifier in the regulator of the electrical system of an internal combustion engine having a flywheel-mounted permanent magnet alternator is well known. Such a regulator provides full wave rectification of the AC input from the alternator to provide a DC output current for charging the battery that supplies the voltage for the DC portion of the system. The bridge rectifier circuit includes a diode or a silicon controlled rectifier (SCR) in each conducting branch Each half cycle of alternator input to the bridge will provide a forward voltage bias to one of the bridge SCRs and the application of a simultaneous gate drive signal to that SCR will allow it to turn on and provide a positive output current from the bridge to the battery. As the current from the alternator to the bridge reverses for the next half cycle, the forward voltage bias is applied to the SCR in the other branch of the bridge simultaneously with a gate drive signal to turn on that SCR and again provide a positive output current to the battery. Thus, the bridge SCR's turn on and begin to conduct only when forward biased and when triggered by an appropriate gate drive signal. The battery is used to provide the source of gate current for the bridge SCR's and, by monitoring battery voltage, gate drive current is supplied to the SCR's only when the battery voltage drops below a detection level, thereby providing battery charge control.
If the regulator is providing DC output current and the battery suddenly becomes disconnected, or if there is an abnormally high impedance between the regulator and the battery or in the battery itself, the regulator output voltage would tend to rise to an undesirable level which might damage or destroy equipment or instruments in the electrical system. This voltage rise is due to the inherently high open circuit voltage of the permanent magnet alternator; whenever the load connected to the regulator becomes unable to accept the regulator output current, the alternator voltage and the regulator output voltage will rise. To avoid damage from a severe voltage surge, overvoltage protection is provided by utilizing a crowbar SCR means connecting each alternator AC input lead to ground. The crowbar SCR triggering circuit monitors the positive voltage appearing on both alternator AC input leads to the bridge and, should an overvoltage condition be detected, the crowbar SCR will be triggered and will then conduct the remaining half cycle of AC input current to ground. Typically, a separate crowbar SCR and associated triggering circuit has been used in each branch of the bridge, connected between an AC input and ground in parallel with one of the bridge SCR's. It is, however, also possible to use a single crowbar SCR and triggering circuit to monitor both AC input lines from the alternator to the SCR/diode bridge.
U.S. Pat. No. 4,410,847 discloses the use of dual crowbar SCR's for overvoltage protection in a conventional four diode bridge rectifier. U.S. Pat. No. 4,431,959 shows a regulator for a permanent magnet alternator using an SCR/diode bridge in which overvoltage protection is provided by a single zener diode. The high power dissipation inherent in a zener diode may not allow such a device to withstand a rapid succession of severe voltage surges.
The attractiveness of using a crowbar SCR for overvoltage protection resides in its inherent low voltage and low power dissipation when switched into a conducting state. As a result, a crowbar SCR experiences much lower energy absorption while performing its protective function than a zener diode and can normally withstand a rapid series of such surges without exceeding its safe temperature. However, an SCR is typically designed for operation at relatively low frequencies, such as 60 hertz, but when used as a protective crowbar SCR in the rectifying bridge for a permanent magnet alternator, the crowbar SCR may experience currents at frequencies which are 10 times greater. Thus, at relatively high engine and alternator speeds, the SCR/diode bridge, and the crowbar SCR protective device when triggered by an overvoltage, may experience full wave rectified current at 600 hertz or higher.
When combined with the characteristics of the permanent magnet alternator, the current passing through the crowbar SCR may then be such that the periods of low current are of insufficient duration to allow the SCR to turn off. Consequently, once the crowbar SCR had been triggered on by an overvoltage, it would be unable to turn off. Additionally, mere repetitive operation of the crowbar SCR in the absence of direct cooling, such as water cooling, which often is not practically available for the crowbar SCR, may cause it to overheat, resulting in the loss of gate control. The crowbar would then begin to turn on solely with the application of forward voltage, effectively staying on continuously. Under either of the above conditions, it is likely that the crowbar SCR would not be able to recover and cool off, and, as a result, quickly burn out.
It would, therefore, be desirable to have some means of assuring an appropriate period of recovery and cool off for the crowbar SCR means, whether utilizing one or two crowbar SCR's, whenever a crowbar is triggered to shunt to ground the current resulting from a transient overvoltage input to the bridge from the alternator.